MOSCOW, RUSSIA - The Kremlin said on Sunday that inflexibility on the part of the United States was to blame for the lack of a bilateral meeting between Russia's President Vladimir Putin and U.S. President Donald Trump during a summit in Vietnam.

Trump and Putin met briefly on the sidelines of the Asia-Pacific Economic Cooperation summit in Vietnam on Saturday and agreed on a joint statement supporting a political solution for Syria, but did not hold substantive bilateral talks.

'Unfortunately the American side did not offer any alternatives despite all efforts of our Russian colleagues.

There was only one time offered that was convenient for the American side, and only one place offered, which had already been rented by the Americans,' Kremlin spokesman Dmitry Peskov was quoted as saying by the RIA news agency.

'The Americans showed no flexibility, and unfortunately did not offer any other alternative proposals. That is why the meeting could not happen,' Peskov added.

Putin himself said on Saturday the lack of a bilateral meeting with Trump in Vietnam was due to both leaders' schedules and protocol obstacles that their teams had been unable to overcome.

Allegations that Trump's election campaign colluded with Moscow last year to turn voters away from Democratic candidate Hillary Clinton have hampered the president's efforts to improve frosty U.S.-Russian relations.

Putin renewed his denial of the allegations during his brief meeting with Trump on Saturday. Trump has previously said the accusations of collusion were a hoax.